A type of flow-rate adjustable valve using a back pressure chamber and a pilot valve body has been used widely because a smaller force is sufficient for moving a main valve body. For example, JP-A-2010-169131 has disclosed the basic structure and the operational principle.
In the above type of flow-rate adjustable valve, when closed, a main valve body provided in a flow path via an elastic member, such as a diaphragm membrane, is biased in a valve-closing direction for closing the main valve body by a pressure of liquid supplied into the back pressure chamber (which pressure depends on a supply pressure in the flow path).
Then, when opened, the pilot valve body is controlled in such a manner that an outlet hole provided in the main valve body is opened. Specifically, an end of the outlet hole on the side of the back pressure chamber is opened. Thereby, the liquid in the back pressure chamber starts to be discharged through the outlet hole. Subsequently, the pressure in the back pressure chamber is reduced, so that the main valve body is opened.
When the liquid in the back pressure chamber starts to be discharged through the outlet hole, new liquid starts to be supplied into the back pressure chamber through an inlet hole provided in the main valve body, for example. As a result, the main valve body is moved and the position of the main valve body is held in such a manner that the amount of the liquid discharged through the outlet hole and the amount of the liquid supplied through the inlet hole are the same (equilibrated). At this time, the amount of the discharged liquid depends on a relative distance between the pilot valve body and the main valve body.
The main valve body is moved according to the above principle. That is to say, the position of the main valve body depends on the position of the pilot valve body. Therefore, by controlling the position of the pilot valve body, the position of the main valve body can be controlled, so that the flow rate of the liquid discharged from the flow-rate adjustable valve can be controlled.